An atomic frequency standard is a device having a resonant system derived from an atomic or molecular specie experiencing a transition between two or more well-defined energy levels of the atom or molecule.
For example, the two lowest energy levels of the rubidium atom (Rb) are known as the ground state hyperfine energy levels A and B. When atoms of gaseous Rb-87 at levels A and B are irradiated with microwave energy at a precise "transition frequency", corresponding to the rubidium frequency that causes atom transitions between the hyperfine energy levels, the rubidium atoms at hyperfine energy level A will make the transition to level B, and vice versa. The transition is employed as a highly accurate frequency reference to which the frequency of a quartz crystal oscillator or voltage-controlled crystal oscillator (VCXO) can be electronically locked in creating an atomic frequency standard.
In such atomic-controlled oscillators the frequency of a quartz crystal oscillator is controlled by means of a physics package and associated electronics that are devoted to maintaining the assigned output frequency, typically 5 MHz or 10 MHz, on a very long-term, exceedingly accurate and stable basis. By properly slaving the quartz crystal oscillator to the frequency of the atomic transition in the physics package, the tendency of the quartz crystal to exhibit drifting due to aging and other inherent as well as environmental effects is markedly suppressed. The physics package of a typical atomic frequency standard generally includes a microwave cavity resonator, an isotopic filter cell, an absorption cell, a light source, a photo detector, temperature control means, and at least one magnetic shield surrounding these components.
In such a typical rubidium atomic frequency standard, for example, the light source is a glass bulb containing rubidium atoms which produces light by an rf-excited plasma discharge. The rubidium in the lamp is heated to a vapor state, approximately 110.degree. C., and is subjected to a high-energy rf field, thereby generating light from the excited rubidium atoms. The "rubidium light" is directed through a filter cell which contains an isotope of rubidium, such as Rb-85, which filters out light with a wavelength that will stimulate transition of atoms from the hyperfine energy level B to any optically-excited level C. The filtered rubidium light is then directed through an absorption cell, also called a resonance cell. The absorption cell includes another isotope of rubidium, Rb-87, and the filtered light energy absorbed by the Rb-87 atoms at hyperfine energy level A causes a transition of the Rb-87 atoms from level A to any optically-excited energy level C. The atoms excited to energy level C, however, do not remain at level C for more than tens of nanoseconds, but return to ground state hyperfine levels A and B in approximately equal numbers by either spontaneous emission of light and/or by collisions, including collisions with other atoms, molecules, or the walls of the absorption cell. Since the filtered light does not allow transitions of atoms from level B to level C, the continuing cycle of optical excitation of atoms from level A to level C and the redistribution of atoms falling from level C between levels A and B eventually results in few, if any, atoms at level A for excitation to level C, and little or no absorption of the light passing through the absorption cell because the atoms have accumulated at hyperfine energy level B. The atoms at level A are said to have been "optically pumped" to level B. If, however, microwave energy is applied to the absorption cell at the rubidium transition frequency, transitions of atoms between hyperfine levels A and B occur, reintroducing atoms at level A which again absorb light energy and undergo a subsequent transition to level C and thereby reduce the light passing through the absorption cell.
The rubidium light passing through the absorption cell is incident on a photo detector, which produces a current output which is proportional to the intensity of the incident light. The current output is processed by servo electronics to provide a control voltage to a voltage controlled crystal oscillator (VCXO) whose output is multiplied (and synthesized) to the rubidium transition frequency and provides the microwave energy used to cause the transitions between hyperfine levels A and B. When the frequency of the microwave energy corresponds to the hyperfine transition frequency, 6.834 GHz for Rb-87, maximum light absorption occurs and the current output of the photo detector is reduced. If, however, the frequency of the microwave energy does not correspond to the hyperfine frequency, then more light will pass through the absorption cell to the photo detector, which in turn increases its current output. Thus, the photo detector current output can be used to provide an error signal to maintain the output frequency of the VCXO, typically 5 or 10 MHz, (which, as noted above, is multiplied and synthesized to produce the hyperfine transition frequency of the rubidium atoms), thereby creating an extremely stable 5 or 10 MHz output frequency standard.
It should be noted in passing that level A is actually three levels, all having the same energy, and level B is actually five levels all having the same energy, but different than the energy of the level A atoms by the hyperfine-energy-level difference. In practice a small static magnetic field (the "C-field" .apprxeq.0.3 Gauss) is applied to the Rb-87 atoms to slightly separate the A levels, and also to slightly separate the B levels. It is well known in the art that microwave energy supplied at the appropriate frequency to an absorption cell containing Rb-87 atoms causes an atomic, or "clock" transition at 6.834 GHz to occur from the single B level with quantum numbers (F=2, M.sub.F =0) to the single A level with quantum numbers (F=1, M.sub.F =0).
Over the years there have been substantial efforts by workers in the field to modify the physics package in order to reduce its overall physical dimensions without changing its operational characteristics.
For example, U.S. Pat. No. 3,903,481 discloses an early development made to reduce the overall size for rubidium vapor frequency standards by eliminating from the physics package one of its elements, the filter cell, that was used to remove one of the hyperfine light frequencies emitted by the light source. This was accomplished by combining the filter cell function with an associated absorption cell function. Thus, the filtering atoms, generally the isotope of rubidium Rb-85, and a buffer gas, were incorporated directly into the absorption cell containing another isotope of rubidium, Rb-87. Therefore, the absorption cell performed dual roles, acting not only as an optically pumped absorption cell to create a population difference for Rb-87 atoms, but, also as a filter cell to avoid unwanted transitions. By combining these two functions within a single cell the physics package was reduced in size.
Other efforts have focused, at least in part, on the excitation of Rb-87 atoms. U.S. Pat. No. 4,405,905 discloses an atomic frequency standard having a microwave loop around the absorption cell. The microwave loop is used to excite Rb-87 atoms contained in an evacuated, wall-coated rubidium absorption cell.
U.S. Pat. No. 4,947,137 discloses a passive atomic frequency standard for interrogating atoms excited by a helical resonator structure, resonant at 6.8 GHz, bonded to the cylindrical surface of the absorption cell.
U.S. Pat. No. 4,494,085 discloses a miniature optical package for an atomic frequency standard having both a filter cell and an absorption cell in the microwave cavity. By positioning the windows of the filter cell and absorption cell all centrally within the microwave cavity, the cavity may be dielectrically loaded and reduced in size. In addition, a Fresnel collimating lens may be placed between the windows of the filter cell and absorption cell and may provide additional dielectric loading of the microwave cavity.